The oxidation of unsubstituted and substituted 6,13-dihydroquinacridone compounds to the corresponding quinacridone is well-known in the art.
For example, numerous publications disclose the oxidation of a 6,13-dihydroquinacridone to the corresponding quinacridone using aromatic nitro compounds as the oxidizing agent in an alcoholic medium containing a small amount of water. However, such processes have the disadvantage of producing considerable organic waste due to the generation of reduced aromatic by-products.
It is also known to oxidize a 6,13-dihydroquinacridone to the corresponding quinacridone by a process wherein the 6,13-dihydroquinacridone is oxidized in a solvent and/or aqueous basic system with an oxygen-containing gas. Such processes are often referred to as "air oxidation" because air is conveniently used as the oxygen containing-gas. Air oxidation processes have the disadvantage that large gas volumes have to be introduced into a heterogeneous reaction mixture, whereby foam is generated. Additionally, it is difficult to determine when the reaction is complete.
Furthermore it is known to oxidize 6,13-dihydroquinacridones dissolved in polar solvents, for example DMSO, using air as the oxidizing agent. Such processes have the advantage of generating excellent quinacridone pigments in a high yield. However, they have the disadvantage of producing a substantial amount of organic waste, such as dimethylsulfone, as by-product during the oxidation reaction, which requires costly solvent regeneration systems.
The present invention is based on the discovery that unsubstituted and substituted quinacridones are prepared by an environmentally friendly process whereby a premilled 6,13-dihydroquinacridone is oxidized to the corresponding quinacridone in an aqueous, basic medium using hydrogen peroxide as the oxidant. It is necessary to premill the 6,13-dihydroquinacridone in order to achieve an acceptable degree of oxidation when hydrogen peroxide is the oxidant.
The inventive process offers the advantage of oxidizing the 6,13-dihydroquinacridone without organic solvents and with only a small excess of base in an aqueous system. The hydrogen peroxide oxidant is additionally advantageous because it does not generate a reduced by-product, but rather oxidizes impurities present in the 6,13-dihydroquinacridone from the presynthesis steps. Thus, the inventive process produces a quinacridone product with improved purity and chroma. Additionally, less water has to be used to wash the presscake of the quinacridone product compared to the known oxidation processes.
In another related aspect, the present invention is based on the discovery that gamma-II quinacridone is obtained, if desired in polymorphically pure form, by an environmentally friendly process whereby finely milled 6,13-dihydroquinacridone is oxidized in a basic, aqueous medium using hydrogen peroxide as oxidant. Since the gamma-II form of quinacridone is widely used, the ability to produce it in polymorphically pure form by an anvironmentally friendly process is a great advantage.